As the ratio of tire noise occupied in automobile noise becomes relatively large accompanied with a further improvement of the silence in recent vehicles, the reduction of such a tire noise is a significant matter. In especial, the tire noise at about 1000 Hz easily hearing with the human ears becomes a main factor of the off-vehicle noise and the prompt countermeasures with respect to this noise are also demanded from a viewpoint of the environmental problems.
In general, it is known that the tire noise at approximately 800-1400 Hz is generated by a columnar resonance resulted from the occurrence of the resonance in an air column defined between a circumferential groove of the tire and a road surface in the ground contact face of the tire. In order to control such a columnar resonance, it is known that the reduction of the groove volume in the circumferential groove is effective.
However, the reduction of the groove volume in the circumferential groove is obliged to lower the drainage property of the tire and hence the wet performance thereof. For the purpose of reducing the tire noise without lowering the wet performance, for example, JP-A-6-143932 proposes a pneumatic tire wherein one wide-width circumferential groove having a groove width of 25-70 mm is formed in a central region of the tread and lateral grooves each opening to a ground contact end of the tread but not opening to the wide-width circumferential groove and having a groove width corresponding to 5-15% of the wide-width circumferential groove are formed at both side portions of the wide-width circumferential groove.
In this proposed tire, however, the large decrease of the ground contact face of the tread necessarily arises because of the existence of the wide-width circumferential groove and also the large stepwise difference of the ground contact pressure in the widthwise direction of the tread can not be avoided so that this tire has a problem that it becomes particularly difficult to simultaneously establish the steering stability and the limit gripping property on a dry road surface.
The invention is to solve such a problem of the conventional technique and to provide a pneumatic tire largely reducing a columnar resonance of the tire while attaining the sufficient establishment of the steering stability and limit gripping property on the dray road surface but also effectively controlling the lowering of the wet performance as well as a method of designing a tread pattern of the tire.